Intercourse, whether between a male and female, two males or two females, involves (1) stimulation of emotions, (2) stimulation of primary sexual areas, (3) stimulation of secondary sexual areas, and, optionally, (4) penetration of one or more orifices. I say optionally, because male masturbation does not include penetration and female masturbation does not always include penetration either.
I will presume, for the sake of this response, that you are not a virgin and have had intercourse with the opposite gender at least once.
Emotional stimulation is generally effected through kissing, looking, sighs, whispers, love words, affection, etc.
Stimulation of primary sexual areas involves direct and indirect contact to the breasts and genitalia, especially the male pons or the female clitoris. Stimulation can be effected using lips, tongue, mouth, hands, other body parts (not limited to genitalia), even breath.
Stimulation of secondary sexual areas generally involves caressing, stroking, licking, massaging, breathing on or rubbing on chest, neck, back, belly, buttocks, thighs, calves, feet, hands, arms, ears, faces, even scalps… basically any part of the body other than the breasts and genitalia. As one’s excitement level increases, sensitivity also increases, making the smallest touch arousing.
Finally, penetration usually, but not always, results in order to achieve orgasm. Orgasm CAN be achieved without penetration, even without being touched, either by oneself and/or one’s partner(s). Penetration between males and females generally involves insertion of the penus into the vagina or rectum. Between two males it usually involves insertion of one male’s penus into the other male’s rectum. Lacking a penus, penetration between two females is generally accomplished either with the hands or an implement, most often a dildo or vibrator, or with a piece of fruit (usually wrapped in plastic and/or coated with a lubricating gel designed for sexual use), most commonly a banana.
Penetration with the hand is seldom 100% penetration. Depending on the size of one woman’s hand in relation to the other woman’s vagina, in most cases only 2–3 fingers can penetrate. The goal is sometimes to get the whole fist in, but the only real advantage is being able to directly stimulate the woman’s G-spot – a location within the vagina of optimum sensitivity. This article may help you understand this topic better: http://en.wikipedia.org/wiki/G-spot
Hand penetration is not limited to female/female intercourse. Male/female intercourse also employs stimulation of the female’s G-spot and/or the male’s G-spot, which involves stimulation of the prostate gland through massage of the area between the bulb of the penus and the rectum. This may help your understand this topic: http://en.wikipedia.org/wiki/Male_G-spot#Male_sexual_response
Hand penetration is probably also used in male/male intercourse, either by stimulation of the male’s G-spot or actual finger penetration of the rectum. Not being a male, myself, and never having had intercourse where one male stimulated another male, I cannot speak from experience here.
However, for the record, I am not a lesbian. I am bi-sexual and yes, I have had intercourse alone with a female and also “threesomes” between myself, a male, and another female. So, yes, I am speaking from experience in every other respect.
Here is an excellent book on how to enhance your sexual interactions, especially for couples experiencing motive-disorders or age-related limitations. It works just fine with perfectly normal, healthy adults, too. Enjoy!
http://www.amazon.com/ESO-Lover-Extended-Sexual-Orgasm/dp/0446677620/ref=pd_bbs_1?ie=UTF8&s=books&qid=1198798430&sr=8–1